Accommodation enables a person of normal vision to focus on objects from infinity to a near point, typically of the order of 25 cm or less from the eye. A presbyope has lost the ability to accommodate over such a large range, and typically requires two corrections: a distance correction for focussing to infinity, and a near correction for focussing to close objects. Corrections, which are typically different for each eye, are usually prescribed to a quarter of a diopter, and may range between approximately +8-−8 diopters for both the distance correction and the near correction.
Multifocal spectacles provide the two corrections for each eye, as well as intermediate corrections, in separate regions of one lens. Such spectacles rely on the fact that the spectacle lenses are relatively fixed with respect to the eye, so that the spectacle wearer usually looks through the higher portions of the lens for distant objects, and the lower portion of the lens for near objects.
Multifocal contact lenses that provide both corrections for a particular eye in a single lens are known in the art. However, unlike spectacle lenses, contact lenses move with the motion of the eye. Thus, multifocal contact lenses known in the art use a number of different systems to enable a presbyope to be adequately corrected for viewing both near and distant objects. Unfortunately, not all of these systems have been well received by users.
U.S. Pat. No. 5,125,729, to Mercure, whose disclosure is incorporated herein by reference, describes a lens which has, on its front surface, a central spherical circular area which is surrounded by an annular aspherical area. The central area is responsible for distance vision. The annular aspherical area enables vision at all distances including reading.
U.S. Pat. No. 5,526,071, to Seidner et al., whose disclosure is incorporated herein by reference, describes a diagnostic contact lens. After diagnosis involving an over-refraction process, the patient is fitted with a prescription lens. The prescription lens has two front annular aspheric surfaces, or in the case where intermediate vision is to be accommodated, three (or more) front aspheric surfaces. The distance vision area is a central area of the lens, while the near vision area is one of the annular areas in the periphery of the front surface of the lens.
U.S. Pat. Nos. 5,619,289 and 5,691,797 to Seidner et al., whose disclosures are incorporated herein by reference, describe a lens having a front central aspheric surface for distance vision correction with a diameter of about 1.5-2.5 mm and an eccentricity of about −0.8. A first annular surface surrounding the central surface is aspheric and has a diameter of about 2.0-3.5 mm. Other annular surfaces have diameters of about 2.3-4.0, and 3.5-8.0, and may be spheric or aspheric.
U.S. Pat. No. 5,754,270 to Rehse et al., whose disclosure is incorporated herein by reference, describes a lens having a central aspheric optic zone with a diopter power equal to the distant power correction, plus an add power range of from 2.5 to 2.25 diopters from center outward. There is a second “blending zone” concentric with the center zone, that provides a rapid power shift of about −0.5 to about −1.25 diopters over a small distance of about 0-0.2 mm, typically about 0.05 mm.
U.S. Pat. Nos. 5,864,379 and 6,540,353 to Dunn, whose disclosures are incorporated herein by reference, describe a lens having a central spherical circular region that is overcorrected for near vision, typically by about 25%-100%, with a diameter of about 1.0-2.5 mm. The central zone is surrounded by up to three annular aspherical zones, the first two having thicknesses of about 0.5 mm, the third having an outer diameter of about 8 mm.
U.S. Pat. Nos. 6,030,077 and 6,260,966 to Sawano et al., whose disclosures are incorporated herein by reference, describe a lens having a central circular spheric region with a first constant power, an intermediate annular aspheric region, and an outer annular spheric region having a second constant power. The radii of different parts of the intermediate region are calculated from two opposing parabolic curves which smoothly connect to each other and to the constant powers of the center and outer zones.
U.S. Pat. No. 6,116,735 to Wada, whose disclosure is incorporated herein by reference, describes a lens having zones which alternate between near- and far-correction zones. Typically there are four zones. A central near-vision zone is approximately 1-2 mm diameter, and the other zones have equal widths of about 0.63 mm.
Correct provision of a multifocal contact lens for a specific eye typically requires knowledge of both the near and far correction of the eye of the patient. Since both corrections are typically prescribed to within a quarter of a diopter, it will be appreciated that there are a large number of near- and far-combinations. Notwithstanding that certain multifocal lenses known in the art, including some of those described above, claim to be suitable for ranges of prescriptions, there is a need for an improved multifocal contact lens.